Conversion of synthesis gas to aromatic hydrocarbons

ABSTRACT

Synthesis gas is converted to aromatic hydrocarbons over an intimate mixture of catalysts comprising a first component of Zno-Cr 2  O 3  mixed catalyst, characterized by catalytic activity for the reduction by hydrogen of carbon monoxide, wherein the Zn.Cr atomic ratio is less than about 4:1 and a second component selected from a selective class of acidic crystalline aluminosilicates having a silica:alumina ratio greater than 12:1 and a pore dimension greater than about 5 Angstroms.

This application contains information related to application Ser. No.730,871 filed Oct. 8, 1976 now U.S. Pat. No. 4,096,163 and applicationSer. No. 566,162, filed Apr. 1975 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is concerned with an improved process for convertingsynthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen orhydrogen donors, to hydrocarbon mixtures. This invention is particularlyconcerned with a process for converting synthesis gas to hydrocarbonmixtures rich in aromatic hydrocarbons. In another aspect, thisinvention is concerned with providing novel catalysts for the conversionof synthesis gas to hydrocarbon mixtures rich in aromatic hydrocarbons.

2. Prior Art

Processes for the conversion of coal and other hydrocarbons such asnatural gas to a gaseous mixture consisting essentially of hydrogen andcarbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen andcarbon monoxide and carbon dioxide, are well known. Although variousprocesses may be employed for the gasification, those of majorimportance depend either on the partial combustion of the fuel with anoxygen-containing gas or on the high temperature reaction of the fuelwith steam, or on a combination of these two reactions. An excellentsummary of the art of gas manufacture, including synthesis gas, fromsolid and liquid fuels, is given in Encyclopedia of Chemical Technology,Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966),Interscience Publishers, New York, New York, the contents of which areherein incorporated by reference. The techniques for gasification ofcoal or other solid, liquid or gaseous fuel are not considered to be perse inventive here.

It would be very desirable to be able to effectively convert synthesisgas, and thereby coal and natural gas, to highly valued hydrocarbonssuch as motor gasoline with high octane number, petrochemicalfeedstocks, liquefiable petroleum fuel gas, and aromatic hydrocarbons.It is well known that synthesis gas will undergo conversion to formreduction products of carbon monoxide, such as hydrocarbons, and/oroxygen containing compounds such as methanol at from about 300° F. toabout 850° F. under from about one to one thousand atmospheres pressure,over a fairly wide variety of catalysts. The Fischer-Tropsch process,for example, which has been most extensively studied, produces a rangeof liquid hydrocarbons, a portion of which have been used as low octanegasoline. The types of catalysts that have been studied for this andrelated processes include those based on metals, oxides or othercompounds of iron, cobalt, nickel, ruthenium, thorium, rhodium andosmium. Methanol synthesis processes, for example, use catalystscomposed of mixtures of two or more oxides and in particular use ZnObase and CuO base mixed catalysts. A review of catalytic processes forthe synthesis of methanol from mixtures containing CO and H₂ is given inEmmett, P.H., Catalysis III, N. Y., Reinhold, 1955. Chapter 8, pages349-411, by G. Natta, Synthesis of Methanol, the text of which isincorporated herein by reference.

The wide range of catalysts and catalyst modifications disclosed in theart and an equally wide range of conversion conditions for the reductionof carbon monoxide by hydrogen provide considerable flexibility towardobtaining selected boiling-range products. Nonetheless, in spite of thisflexibility, it has not proved possible to make such selections so as toproduce liquid hydrocarbons in the gasoline boiling range which containhighly branched paraffins and substantial quantities of aromatichydrocarbons, both of which are desired for high quality gasoline, or toselectively produce aromatic hydrocarbons particularly rich in thebenzene to xylenes range. A review of the status of this art is given in"Carbon Monoxide-Hydrogen Reactions", Encyclopedia of ChemicalTechnology, Edited by Kirk-Othmer, Second Edition, Volume 4, pages446-488 and and Volume 13, pages 370-398. Interscience Publishers, NewYork, N. Y., the text of which is incorporated herein by reference.

Recently it has been discovered that synthesis gas may be converted tooxygenated organic compounds and these then converted to higherhydrocarbons, particularly high octane gasoline, by catalytic contact ofthe synthesis gas with a carbon monoxide reduction catalyst followed bycontacting the conversion products so produced with a special type ofzeolite catalyst in a separate reaction zone. This two-stage conversionis described in copending U.S. patent application, Ser. No. 387,220,filed on Aug. 9, 1973.

Still more recently, it has been discovered that synthesis gas may beconverted to hydrocarbon mixtures useful in the manufacture of heatingoils, gasoline, aromatic hydrocarbons, and chemical intermediates bycatalytic contact with an intimate mixture of: (1) carbon monoxidehydrogen reduction catalyst comprising a methanol synthesis catalyst and(2) a special type of zeolite catalyst comprising an acidic crystallinealuminosilicate having a silica:alumina ratio greater than 12 and a poredimension greater than about 5 Angstroms. This one-stage conversion isdescribed in copending U.S. Patent application Ser. No. 730,871, filedon Oct. 8, 1976.

It is an object of the present invention to provide an improved processfor converting fossil fuels to a hydrocarbon mixture that contains largequantities of highly desirable constituents. It is a further object ofthis invention to provide a more efficient method for converting amixture of gaseous carbon oxides and hydrogen to a mixture ofhydrocarbons. It is a further object of this invention to provide animproved method for converting synthesis gas to a hydrocarbon mixturerich in aromatic hydrocarbons. It is a further object of this inventionto provide novel catalysts for the conversion of synthesis gas to ahydrocarbon mixture rich in aromatic hydrocarbons.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that valuable hydrocarbon mixtures may beproduced by reacting synthesis gas, i.e., mixtures of hydrogen gas withgaseous carbon oxides, or the equivalents of such mixtures, in thepresence of certain heterogeneous catalysts comprising intimate mixturesof at least two components. Copending U.S. Patent Application Ser. No.730,871 (referred to above) discloses the selective production of lightparaffins from synthesis gas using catalysts comprising a methanolsynthesis component and an acidic crystalline aluminosilicate componentof selected characteristics. The present invention is based on thefurther discovery that a class of chromium oxide catalyst with orwithout the presence of zinc oxide and referred to as a methanolsynthesis catalysts, as will be more fully described hereinafter, may bemodified such that aromatics are produced when this catalyst is used inconjunction with an acidic crystalline aluminosilicate catalyst ofselected characteristic.

It has now been found that aromatics can be synthesized from syngassimply by shifting the metals ratio of a zinc oxide-chromia catalystaway from an optimum for methanol synthesis; specifically, this isaccomplished by providing a Zn:Cr ratio less than about 4:1. Whencombined with an acidic crystalline aluminosilicate component hereindefined, the catalyst will convert synthesis gas to a mixture of aboutequal parts of LPG and aromatics with minimal methane production. Thisintimate catalyst mixture thus produces highly desirable aromaticproducts with good selectivity and does so with extraordinarily highconversion per passs. Furthermore, when the preferred class of acidiccrystalline aluminosilicate component is used in the intimate mixture,catalytic activity is sustained for unusually long periods of time andthe aromatic hydrocarbons produced are rich in toluene and xylene.Finally, the catalyst of this invention is air regenerable and has shiftcapability.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

Synthesis gas for use in this invention consists of a mixture ofhydrogen gas with gaseous carbon oxides including carbon monoxide andcarbon dioxide. By way of illustration, a typical purified synthesis gaswill have the composition, on a water-free basis, in volume percentages,as follows: hydrogen, 51; carbon monoxide, 40; carbon dioxide, 4;methane, 1; and nitroen, 4.

The synthesis gas may be prepared from fossil fuels by any of the knownmethods, including such in situ gasification processes as theunderground partial combustion of coal and petroleum deposits. The termfossil fuels, as used herein, is intended to include anthracite andbituminous coal, lignite, crude petroleum, shale oil, oil from tarsands, natural gas, as well as fuels derived from simple physicalseparations or more profound transformations of these materials,including coked coal, petroleum coke, gas oil, residua from petroleumdistillation, and two or more of any of the foregoing materials incombination. Other carbonaceous fuels such as peat, wood and cellulosicwaste materials also may be used.

The raw synthesis gas produced from fossil fuels will contain variousimpurities such as particulates, sulfur, and metal carbonyl compounds,and will be characterized by a hydrogen-to-carbon oxides ratio whichwill depend on the fossil fuel and the particular gasificationtechnology utilized. In general, it is desirable for the efficiency ofsubsequent conversion steps to purify the raw synthesis gas by theremoval of impurities. Techniques for such purification are knwon andare not part of this invention. It is preferred to adjust thehydrogen-to-carbon oxides volume ratio to be within the range of from0.2 to 6.0 prior to use in this invention. Should the purified synthesisgas be excessively rich in carbon oxides, it may be brought within thepreferred range by the well-known water-gas shift reaction. On the otherhand, should the synthesis gas be excessively rich in hydrogen, it maybe adjusted into the preferred range by the addition of carbon dioxideor carbon monoxide. Purified synthesis gas adjusted to contain a volumeratio of hydrogen-to-carbon oxides of from 0.2 to 6.0 will be referredto as adjusted synthesis gas.

It is contemplated that the synthesis gas for use in this inventionincludes art-recognized equivalents to the already described mixtures ofhydrogen gas with gaseous carbon oxides. Mixtures of carbon monoxide andsteam, for example, or of carbon dioxide and hydrogen, to provideadjusted synthesis gas by in situ reaction, are contemplated.

The heterogeneous catalysts of this invention comprise at least twocomponents intimately mixed, and in which one component is selected fromthe class of ZnO-Cr₂ O₃ substances that have catalytic activity for thereduction by hydrogen of carbon monoxide wherein the Zn:Cr atomic ratiois less than than about 4:1, and in which the other component is a classof acidic acidic crystalline aluminosilicate characterized by a poredimension greater than about 5 Angstroms, a silica-to-alumina ratio ofat least 12 and a constraint index within the range of 1 to 12.

The ZnO-Cr₂ O₃ substance or component characterized by catalyticactivity for the reduction by hydrogen of carbon monoxide may beselected from any of the art-recognized ZnO-Cr₂ O₃ mixed catalysts forproducing hydrocarbons, oxygenated products, or mixtures thereof, fromsynthesis gas, subject only to the further restriction that the Zn:Crratio of the mixed catalyst systems be less than about 4:1. Preferably,the Zn:Cr atomic ratio is within the range from about 3.8:1 to 0:1.Examples of these mixed catalyst systems include mechanical mixtures ofZnO and Cr₂ O₃, mixtures of ZnO.Cr₂ O₃ (zinc chromite) and ZnO, calcinedmixtures of coprecipitated zinc and chromium hydroxides or carbonates,and thermally decomposed mixtures of zinc and chromium acetates.

The ZnO-Cr₂ O₃ mixed catalyst component should in all cases constitutefrom 0.05 to 99 percent by weight, and preferably from 1 percent to 95percent of the intimate mixture. The ZnO-Cr₂ O₃ mixed catalyst componentmay be furnished as elemental metals or as corresponding metalcompounds.

The acidic crystalline aluminosilicate component of the heterogeneouscatalyst is characterized by a pore dimension greater than about 5Angstroms, i.e., it is capable of sorbing paraffins having a singlemethyl branch as well as normal paraffins, and it has asilica-to-alumina ratio of at least 12. Zeolite A, for example, with asilica-to-alumina ratio of 2.0 is not useful in this invention, it hasno pore dimension greater than about 5 Angstroms.

The crystalline aluminosilicates herein referred to, also known aszeolites, constitute an unusual class of natural and synthetic minerals.They are characterized by having a rigid crystalline framework structurecomposed of an assembly of silicon and aluminum atoms, each surroundedby a tetrahedron of shared oxygen atoms, and a precisely defined porestructure. Exchangeable cations are present in the pores.

The catalysts referred to herein utilize members of a special class ofzeolites exhibiting some unusual properties. These zeolites induceprofound transformations of aliphatic hydrocarbons to aromatichydrocarbons in commercially desirable yields and are generally highlyeffective in alkylation, isomerization, disproportionation and otherreactions involving aromatic hydrocarbons. Although they have unusuallylow alumina contents, i.e. high silica to alumina ratios, they are veryactive even with silica to alumina ratios exceeding 30. This activity issurprising since catalytic activity of zeolites is generally attributedto framework aluminum atoms and cations associated with these aluminumatoms. These zeolites retain their crystallinity for long periods inspite of the presence of steam even at high temperatures which induceirreversible collapse of the crystal framework of other zeolites, e.g.of the X and A type. Furthermore, carbonaceous deposits, when formed,may be removed by burning at higher than usual temperatures to restoreactivity. In many environments the zeolites of this class exhibit verylow coke forming capability, conducive to very long times on streambetween burning regenerations.

An important characteristic of the crystal structure of this class ofzeolites is that it provides constrained access to, and egress from, theintra-crystalline free space by virtue of having a pore dimensiongreater than about 5 Angstroms and pore windows of about a size such aswould be provided by 10-membered rings of oxygen atoms. It is to beunderstood, of course, that these rings are those formed by the regulardisposition of the tetrahedra making up the anionic framework of thecrystalline aluminosilicate, the oxygen atoms themselves being bonded tothe silicon or aluminum atoms at the centers of the tetrahedra. Briefly,the preferred zeolites useful in type B catalysts in this inventionpossess, in combination: a silica to alumina ratio of at least about 12;and a structure providing constrained access to the crystalline freespace.

The silica to alumina ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although zeolites with a silica to aluminaratio of at least 12 are useful it is preferred to use zeolites havinghigher ratios of at least about 30. Such zeolites, after activation,acquire an intracrystalline sorption capacity for normal hexane which isgreater than that for water, i.e., they exhibit "hydrophobic"properties. It is believed that this hydrophobic character isadvantageous in the present invention.

The zeolites useful as catalysts in this invention freely sorb normalhexane and have a pore dimension greater than about 5 Angstroms. Inaddition, their structure must provide constrained access to some largermolecules. It is sometimes possible to judge from a known crystalstructure whether such constained access exists. For example, if theonly pore windows in a crystal are formed by 8-membered rings oxygenatoms, then access by molecules of larger cross-section than normalhexane is substantially excluded and the zeolite is not of the desiredtype. Zeolites with windows of 10-membered rings are preferred, althoughexcessive puckering or pore blockage may render these zeolitessubstantially ineffective. Zeolites with windows of twelve-memberedrings do not generally appear to offer sufficient constraint to producethe advantageous conversions desired in the instant invention, althoughstructures can be conceived, due to pore blockage or other cause, thatmay be operative.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access, a simpledetermination of the "constraint index" may be made by continuouslypassing a mixture of equal weight of normal hexane and 3-methylpentaneover a small sample, approximately 1 gram or less, of zeolite atatmospheric pressure according to the following procedure. A sample ofthe zeolite, in the form of pellets or extrudate, is crushed to aparticle size about that of coarse sand and mounted in a glass tube.Prior to testing, the zeolite is treated with a stream of air at 1000°F. for at least 15 minutes. The zeolite is then flushed with helium andthe temperature adjusted between 550° F. and 950° F. to give an overallconversion between 10 percent and 60 percent. The mixture ofhydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volumeof liquid hydrocarbon per volume of catalyst per hour) over the zeolitewith a helium dilution to give a helium to total hydrocarbon mole ratioof 4:1. After 20 minutes on stream, a sample of the effluent is takenand analyzed, most conveniently by gas chromatography, to determine thefraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Catalysts suitable for the presentinvention are those which employ a zeolite having a constraint indexfrom 1.0 to 12.0. Constraint Index (CI) values for some typical zeolitesincluding some not within the scope of this invention are:

    ______________________________________                                        CAS                      C.I.                                                 ______________________________________                                        Erionite                  38                                                  ZSM-5                    8.3                                                  ZSM-11                   8.7                                                  ZSM-35                   6.0                                                  TMA Offretite            3.7                                                  ZSM-38                   2.0                                                  ZSM-12                   2.                                                   Beta                     0.6                                                  ZSM-4                    0.5                                                  Acid Mordenite           0.5                                                  REY                      0.4                                                  Amorphous Silica-alumina 0.6                                                  ______________________________________                                    

The above-described Constraint Index is an important and even critical,definition of those zeolites which are useful to catalyze the instantprocess. The very nature of this parameter and the recited technique bywhich it is determined, however, admit of the possibility that a givenzeolite can be tested under somewhat different conditions and therebyhave different constraint indexes. Constraint Index seems to varysomewhat with severity of operation (conversion). Therefore, it will beappreciated that it may be possible to so select test conditions toestablish multiple constraint indexes for a particular given zeolitewhich may be both inside and outside the above defined range of 1 to 12.

Thus, it should be understood that the parameter and property"Constraint Index" as such value is used herein is an inclusive ratherthan an exclusive value. That is, a zeolite when tested by anycombination of conditions within the testing definition set forth hereinabove to have a constraint index of 1 to 12 is intended to be includedin the instant catalyst definition regardless that the same identicalzeolite tested under other defined conditions may give a constraintindex value outside of 1 to 12.

The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11,ZSM-12, ZSM-35 and ZSM-38, and other similar materials. Recently issuedU.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporatedherein by reference.

ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, theentire contents of which are incorporated herein by reference.

ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449, theentire contents of which are incorporated herein by reference.

The subject of ZSM-35 is described in U.S. application Ser. No. 528,061filed Nov. 29, 1974. The subject of ZSM-38 is described in U.S.application Ser. No. 528,060 filed Nov. 29, 1974.

The specific zeolites described, when prepared in the presence oforganic cations, are substantially catalytically inactive, possiblybecause the intracrystalline free space is occupied by organic cationsfrom the forming solution. They may be activated by heating in an inertatmosphere at 1000° F. for one hour, for example, followed by baseexchange with ammonium salts followed by calcination at 1000° F. in air.The presence of organic cations in the forming solution may not beabsolutely essential to the formation of this special type zeolite;however, the presence of these cations does appear to favor theformation of this special type of zeolite. More generally, it isdesirable to activate this type zeolite by base exchange with ammoniumsalts followed by calcination in air at about 1000° F. for from about 15minutes to about 24 hours.

Natural zeolites may sometimes be converted to this type zeolite byvarious activation procedures and other treatments such as baseexchange, steaming, alumina extraction and calcination, alone or incombinations. Natural minerals which may be so treated includeferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulanditeand clinoptilolite. The preferred crystalline aluminosilicates areZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38, with ZSM-5 particularlypreferred.

The zeolites used as catalysts in this invention may be in the hydrogenform or they may be base exchanged or impregnated to contain ammonium ora metal cation complement. It is desirable to calcine the zeolite afterbase exchange. The metal cations that may be present include any of thecations that may be present include any of the cations of the metals ofGroups I through VIII of the Periodic Table. However, in the case ofGroup IA metals, the cation content should in no case be so large as tosubstantially eliminate the activity of the zeolite for the catalysisbeing employed in the instant invention. For example, a completelysodium exchanged H-ZSM-5 appears to be largely inactive for shapeselective conversions required in the present invention.

In a preferred aspect of this invention, the zeolites useful ascatalysts herein are selected as those having a crystal frameworkdensity, in the dry hydrogen form, of not substantially below about 1.6grams per cubic centimeter. It has been found that zeolites whichsatisfy all three of these criteria are most desired. Therefore, thepreferred catalysts of this invention are those comprising zeolitehaving a constraint index as defined above of about 1 to 12, a silica toalumina ratio of at least about 12 and a dried crystal density of notsubstantially less than about 1.6 gram per cubic centimeter. The drydensity for known structures may be calculated from the number ofsilicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g., onpage 19 of the article on Zeolite Structure by W. M. Meier. This paper,the entire contents of which are incorporated herein by reference, isincluded in "Proceedings of the Conference Molecular Sieves, London,April 1967", published by the Society of Chemical Industry, London,1968. When the crystal structure is unknown, the crystal frameworkdensity may be determined by classical pyknometer techniques. Forexample, it may be determined by immersing the dry hydrogen form of thezeolite in an organic solvent which is not sorbed by the crystal. It ispossible that the unusual sustained activity and stability of this classof zeolites is associated with its high crystal anionic frameworkdensity of not less than about 1.6 grams per cubic centimeter. This highdensity of course must be associated with a relatively small amount offree space within the crystal, which might be expected to result in morestable structures. This free space, however, seems to be important asthe locus of catalytic activity.

Crystal framework densities of some typical zeolites including somewhich are not within the purview of this invention are:

    ______________________________________                                                      Void          Framework                                         Zeolite       Volume        Density                                           ______________________________________                                        Ferrierite    0.28 cc/cc    1.76 g/cc                                         Mordenite     .28           1.7                                               ZSM-5, -11    .29           1.79                                              Dachiardite   .32           1.72                                              L             .32           1.61                                              Clinoptilolite                                                                              .34           1.71                                              Laumontite    .34           1.77                                              ZSM-4 (Omega) .38           1.65                                              Heulandite    .39           1.69                                              P             .41           1.57                                              Offretite     .40           1.55                                              Levynite      .40           1.54                                              Erionite      .35           1.51                                              Gmelinite     .44           1.46                                              Chabazite     .47           1.45                                              A             .5            1.3                                               Y             .48           1.27                                              ______________________________________                                    

The intimate mixture of heterogeneous catalysts may be prepared invarious ways. The two components may be separately prepared in the formof catalyst particles such as pellets or extrudates, for example, andsimply mixed in the required proportions. The particle size of theindividual component particles may be quite small, for example, fromabout 20 to about 150 microns, when intended for use in fluid bedoperation; or they may be as large as up to about 1/2 inch for fixed bedoperation. Or, the two components may be mixed as powders and formedinto pellets or extrudate, each pellet containing both components insubstantially the required proportions. Binders such as alumina,zirconia, silica, titania, magnesia, etc., may be present. Alumina isparticularly preferred because, as shown by the Examples, it has adesirable catalytic effect on the synthesis gas conversion.Alternatively, the ZnO-Cr₂ O₃ component that has catalytic activity forthe reduction of carbon monoxide may be formed on the acidic crystallinealuminosilicate component by conventional means such as impregnation ofthat solid with salt solution of the desired metals, followed by dryingand calcination. Base exchange of the acidic crystalline aluminosilicatecomponent also may be used in some selected cases to effect theintroduction of part or all of the carbon monoxide reduction component.Other means for forming the intimate mixture may be used, such as:precipitation of the carbon monoxide reduction component in the presenceof the acidic crystalline aluminosilicate; or electroless deposition ofmetal on the zeolite; or deposition of metal from the vapor phase.Various combinations of the above preparative methods will be obvious tothose skilled in the art of catalyst preparation. It should becautioned, however, to avoid techniques likely to reduce thecrystallinity of the acidic crystalline aluminosilicate.

It will be recognized from the foregoing description that theheterogeneous catalysts, i.e., the above-described intimate mixtures,used in the process of this invention, may have varying degrees ofintimacy. At one extreme, when using 1/2 inch pellets of the ZnO-Cr₂ O₃carbon monoxide reducing component mixed with 1/2 inch pellets of theacidic crystalline aluminosilicate, substantially all locations withinat least one of the components will be within not more than about 1/2inch of some of the other component, regardless of the proportions inwhich the two components are used. With different sized pellets, e.g.,1/2 inch and 1/4 inch, again substantially all locations within at leastone of the components will be within not more than about 1/2 inch of theother component. These examples illustrate the lower end of the degreeof intimacy required for the practice of this invention. At the otherextreme, one may ball mill together acid crystalline aluminosilicateparticles of about 0.1 micron particle size with chromia with or withoutzinc oxide of similar particle size followed by pelletization. For thiscase, substantially all the locations within at least one of thecomponents will be within not more than about 0.1 micron of some of theother component. This exemplifies about the highest degree of intimacythat is practical. The degree of intimacy of the physical mixture mayalso be expressed as the minimum distance of separation of the centralpoints located within the particles of the two components. This will, onaverage, be represented by one-half the sum of the average particle sizefor the two components. Thus, for the foregoing example illustrating thehighest degree of intimacy, the centers of the particles of either ofthe two components will be separated from the nearest particle of theother component by an average distance of at least about 0.1 micron. Thedegree of intimacy of the heterogeneous catalyst is largely determinedby its method of preparation, but it may be independently verified byphysical methods such as visual observations, examination in an ordinarymicroscope or with an electron microscope, or by electron microprobeanalysis.

In the process of this invention, synthesis gas is contacted with theheterogeneous catalyst at a temperature in the range of from about 400°F. to about 1000° F., preferably from about 500° F. to about 900° F., ata pressure in the range of from about 1 to about 1000 atmospheres,preferably from about 10 to about 300 atmospheres, and at a volumehourly space velocity in the range of from about 500 to about 50,000volumes of gas, at standard temperature and pressure per volume ofcatalyst, or equivalent contact time if a fluidized bed is used. Theheterogeneous catalyst may be contained as a fixed bed, or a fluidizedbed may be used. The product stream containing hydrocarbons, unreactedgases and steam may be cooled and the hydrocarbons recovered by any ofthe techniques known in the art, which techniques do not constitute partof this invention. The recovered hydrocarbons may be further separatedby distillation or other means to recover benzene, toluene, xylenes, orother aromatic hydrocarbons.

EXAMPLES 1-13

Synthesis gas having a H₂ /CO ratio of 1 was reacted at 1200 psig, 800°F., and about 1 WHSV over a series of catalysts. The catalysts wereprepared by coprecipitation of zinc-chromium nitrate solutions with NH₃.The catalysts containing alumina (Examples 1-6 and 10-13) were preparedby introducing alumina to the solution as the nitrate or acetate beforeprecipitation. The precipitates were washed, dried at 100° C., calcinedin air over night at 538° C., combined with HZSM-5, and then pelletized.Exposing the prepared catalyst to flowing gas (H₂ /CO=1) at operatingconditions (1200 psig, 800° F.) overnight was sufficient for activation.Results and catalyst compositions are described in Table I.

In Examples 1-4 and 6 (see Table I), the Zn to Cr ratio was varied whilemaintaining a constant Cr to Al ratio. Conversions and selectivities areplotted against the atom percent of Zn and Cr (Al- and ZSM-5-free basis)in FIG. 1. The composition of a typical commercial zinc chromitemethanol synthesis catalyst (Harshaw Zn - 0302) is also indicated onFIG. 1. It can be seen from the plot that aromatics selectivity is astrong function of the Zn/Cr ratio, and that virtually no aromatics areformed at Zn/Cr ratios optimized for methanol synthesis. As the Crcontent increases, aromatics content rises sharply and passes through amaximum. An inverse correlation of conversion and aromatics selectivityis evident, indicating that aromatization is slower than the competitivehydrogenation reactions under the reaction conditions. Only traces ofmethanol or dimethyl ether were detected in the product. A typicalaromatics distribution is shown in Table II; the overall distribution is84.4 percent A₆ -A₁₀ (8.5 percent durene) and 15.6 percent A₁₁ ⁺. It isbelieved that the A₁₁ ⁺ fraction can be substantially reduced bysubstituting ZSM-11 for ZSM-5 in the catalyst composition.

The effect of changing the Zn/Cr ratio at constant Al can be seen fromthe results of Examples 4, 11, and 12 in Table I. The effect is almostidentical to that shown in FIG. 1 for the relationship between the Zn/Crratio and the Cr/Al ratio.

Examples 7-9 in Table I show the effect of the Zn/Cr ratio in theabsence of Al. FIG. 2 is a plot of conversions and selectivities as afunction of the atom percent of Zn and Cr (again on an Al- and ZSM-5free basis). Completely different correlations from those of FIG. 1 areevident but indicate that Al has a catalytic function and is not simplyan inert diluent.

The effect of Al content at a constant Zn/Cr ratio is shown by Examples4, 8, 10, and 13 and is plotted in FIG. 3. The aromatic selectivitymaximum is again apparent and is further evidence that Al plays anactive role.

Air regenerability of the catalyst composition of this invention wasqualitatively demonstrated on one catalyst sample: the spent catalystfrom Example 4 was placed in a muffle furnace and calcined in air at1000° F. overnight. The catalyst was retested (see Example 5, Table I)and showed a 5 percent loss in CO conversion activity.

                                      TABLE I                                     __________________________________________________________________________    Syngas Conversion Over ZnCrZSM-5 - Effect of Alumina                          Example   1   2   3   4   5    6   7   8   9   10  11  12  13                 Run LPA - 282 A                                                                             291 A                                                                             294 A                                                                             302 A                                                                             302-R1A                                                                            300 A                                                                             257 A                                                                             313 A                                                                             299 A                                                                             319 A                                                                             316 A                                                                             317                                                                               318                __________________________________________________________________________                                                               A                  Catalyst Comp'N, Wt                                                           ZnO       0.03                                                                              0.10                                                                              0.16                                                                              0.27                                                                              0.27 0.61                                                                              0   0.36                                                                              0.70                                                                              0.31                                                                              0.53                                                                              0.04                                                                              0.11               Cr.sub.2 O.sub.3                                                                        0.52                                                                              0.48                                                                              0.44                                                                              0.41                                                                              0.41 0.17                                                                              0.83                                                                              0.54                                                                              0.20                                                                              0.48                                                                              0.15                                                                              0.64                                                                              0.28               Al.sub.2 O.sub.3                                                                        0.35                                                                              0.32                                                                              0.30                                                                              0.22                                                                              0.22 0.12                                                                              0   0   0   0.11                                                                              0.22                                                                              0.22                                                                              0.44               ZSM-5     0.10                                                                              0.10                                                                              0.10                                                                              0.10                                                                              0.10 0.10                                                                              0.17                                                                              0.10                                                                              0.10                                                                              0.10                                                                              0.10                                                                              0.10                                                                              0.10               Reaction Conditions                                                           T°F.                                                                             --  --  --  --  --   800 --  --  --  --  --  --  --                 P, psig   --  --  --  --  --   1200                                                                              --  --  --  --  --  --  --                 H.sub.2 /CO                                                                             --  --  --  --  --   1   --  --  --  --  --  --  --                 GHSV, hr.sup.-1                                                                         1490                                                                              1690                                                                              1690                                                                              1830                                                                              1820 2050                                                                              1680                                                                              1840                                                                              2845                                                                              2020                                                                              2450                                                                              1750                                                                              1840               WHSV, hr.sup.-1                                                                         1.0 1.1 1.0 1.1 1.1  1.1 1.0 1.2 1.8 1.0 1.2 0.9 1.0                TOS, hr   19  19  19  19  20   16  19  18  19  20  19  12  20                 Conversion, %                                                                 CO        66.4                                                                              55.9                                                                              46.9                                                                              41.6                                                                              36.4 84.6                                                                              23.5                                                                              74.9                                                                              79.8                                                                              67.1                                                                              79.2                                                                              54.3                                                                              39.6               H.sub.2   26.3                                                                              34.7                                                                              31.2                                                                              23.2                                                                              24.2 62.4                                                                              21.4                                                                              56.5                                                                              64.6                                                                              51.3                                                                              51.6                                                                              32.1                                                                              15.3               HC YIELD, % C                                                                           37.0                                                                              26.6                                                                              23.3                                                                              17.8                                                                              16.1 45.8                                                                              16.2                                                                              39.4                                                                              42.8                                                                              35.7                                                                              42.3                                                                              25.1                                                                              19.1               HC                                                                            Distribution, Wt %                                                            Methane   2.1 3.1 3.3 3.4 2.3  6.7 2.0 3.1 7.0 3.2 7.4 3.2 4.2                Ethane    26.5                                                                              15.6                                                                              15.1                                                                              12.6                                                                              16.2 15.1                                                                              12.2                                                                              8.8 15.4                                                                              9.7 10.7                                                                              10.9                                                                              9.6                Ethylene  0.1 0.2 0.2 0.2 0.3  0.2 0.3 0.1 0.1 0.1 0.2 --  0.2                Propane   13.2                                                                              15.8                                                                              15.0                                                                              15.6                                                                              13.1 17.5                                                                              8.8 26.3                                                                              13.6                                                                              28.9                                                                              14.9                                                                              27.6                                                                              15.1               Propylene <0.1                                                                              0.1 0.1 0.1 0.1  0.1 0.1 0.1 0.1 0.1 0.1 --  0.1                Butanes   6.1 6.6 6.0 5.6 4.7  27.0                                                                              4.4 26.9                                                                              21.6                                                                              26.7                                                                              25.4                                                                              13.1                                                                              7.2                Butenes   --  --  --  --  --   --  --  --  --  --  --  --  --                 C.sub.5.sup.+ PON                                                                       10.3                                                                              4.4 3.7 4.1 3.3  27.1                                                                              2.5 18.6                                                                              33.9                                                                              16.3                                                                              34.2                                                                              6.5 4.5                Aromatics 41.7                                                                              54.2                                                                              56.6                                                                              58.4                                                                              60.0 6.3 69.7                                                                              16.1                                                                              8.3 15.0                                                                              7.1 38.7                                                                              59.1               __________________________________________________________________________

                  TABLE II                                                        ______________________________________                                        Aromatics Distribution                                                        Example 4                                                                                          Wt. %                                                    ______________________________________                                        Benzene                <0.1                                                   Toluene                2.0                                                    Ethylbenzene           0.1                                                    Xylenes                12.1                                                   Tri Me benzenes                                                                1, 2, 3               3.4                                                     1, 2, 4               26.3                                                    1, 3, 5               10.1                                                   Tetra Me benzenes                                                              1, 2, 3, 4            3.6                                                     1, 2, 3, 5            17.0                                                    1, 2, 4, 5 (durene)   8.5                                                    Other A.sub.10         1.3                                                    A.sub.11.sup.+         15.6                                                                          100.0                                                  ______________________________________                                    

EXAMPLES 14-16

These examples are identical to the previous Examples except thatzirconia was substituted for alumina as the binder for the intimatecatalyst mixture. Results and catalyst compositions are shown in TableIII and FIG. 4 is a plot of the results. The plot is similar to FIG. 2(Al-free catalysts) and indicates that ZrO₂ has little or no catalyticeffect on the zinc chromite catalyst.

                  TABLE III                                                       ______________________________________                                        Syngas Conversion over ZnCr ZSM-5 - Effect of Zirconia                        RUN LPA -         308 A    307 A    305 B                                     ______________________________________                                        CATALYST COMP'N, PTS.                                                         ZnO            0.03        0.46     0.21                                      Cr.sub.2 O.sub.3                                                                             0.56        0.13     0.38                                      ZrO.sub.2      0.31        0.31     0.31                                      ZSM-5          0.10        0.10     0.10                                      REACTION CONDITIONS                                                           T°,F.   --          800      --                                        P, psig        --          1200     --                                        H.sub.2 /CO    --          1        --                                        GHSV, hr.sup.-1                                                                              1670        2880     2320                                      WHSV, hr.sup.-1                                                                              1.1         1.4      1.0                                       TOS, hr        18          18       24                                        CONVERSION, %                                                                 CO             61.8        83.0     71.6                                      H.sub.2        36.2        68.0     50.7                                      HC YIELD, % C  34.2        45.9     53.5                                      HC DISTRIBUTION, WT %                                                         Methane        3.0         5.9      3.0                                       Ethane         19.7        13.1     11.7                                      Ethylene       0.1         0.2      0.1                                       Propane        19.8        13.6     21.2                                      Propylene      0.1         0.2      0.1                                       Butanes        9.1         22.1     24.3                                      Butenes        --          --       --                                        C.sub.5.sup.+ PON                                                                            6.7         37.2     20.2                                      Aromatics      41.5        7.7      19.4                                      ______________________________________                                    

What is claimed is:
 1. In the process of converting synthesis gas,comprising carbon monoxide and hydrogen, to an aromatic hydrocarbonproduct by contacting the synthesis gas at about 500° to 900° F. with aheterogenous catalyst comprising as a first component a carbon monoxidereduction catalyst and as a second component an aromatizing crystallinealuminosilicate zeolite having a silica to alumina ratio of at least 12and a constraint index of about 1 to 12; the improvement which comprisesutilizing as the carbon monoxide reducing catalyst component a mixedcatalyst selected from the groups consisting of ZnO-Cr₂ O₃ having aZn:Cr atomic ratio less than about 4:1 alone or in admixture with abinder selected from the group consisting of alumina, zirconia, silica,titania and magnesia and thereby substantially increasing the conversionof said synthesis gas to aromatic hydrocarbon products.
 2. The method ofclaim 1 wherein the ZnO-Cr₂ O₃ mixed catalyst component comprises fromabout 1 to about 95 weight percent of the heterogeneous catalystmixture.
 3. The method of claim 1 wherein the Zn:Cr atomic ratio of theZnO-Cr₂ O₃ mixed catalyst component is within the range from about 3.8:1to 0:1.
 4. The method of claim 1 wherein said ZnO-Cr₂ O₃ component andsaid crystalline aluminosilicate are in the same particle.
 5. The methodof claim 1 wherein said ZnO-Cr₂ O₃ component and said crystallinealuminosilicate are in separate particles.
 6. The method of claim 1wherein said crystalline aluminosilicate is selected from a class ofcrystalline zeolites comprising ZSM-5, ZSM-11, ZSM-12, ZSM-35 andZSM-38.
 7. The method of claim 1 wherein the synthesis gas hydrogen tocarbon oxides volume ratio is maintained within the range of from0.2-6.0.
 8. The method of claim 1 wherein the binder is alumina.